Knickpoint Migration: Processes, Rates, and Form

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Knickpoint migration has long been recognized as a primary mechanism of river response to sudden base-level fall. As base-level fall can be triggered by tectonic upheaval, climatic change, sea-level fall or river capture, knickpoint migration is an important aspect of channel incision and landscape evolution in a wide range of geological, tectonic, and climatic settings. Following a sudden base-level fall, the distribution of actively migrating knickpoints within a watershed sets the boundary between landforms that have adjusted to the new base level and those that have not, and therefore retain their relict, pre-incision form. Consequently, the time scale of landscape response and the timing of sediment deliver to basins following a change in tectonic or climatic conditions is strong influenced by the rate and style of upstream knickpoint migration.Despite the critical importance of knickpoints in landscape evolution, little is known quantitatively about the controls on the rates and styles of knickpoint migration. We propose to exploit a natural experiment in knickpoint migration that has been running for 18 ka in the Waipaoa drainage basin on the East Cape of New Zealand's North Island. Over on hundred knickpoints demarcate the headward advance of a wave of incision initiated along the Waipaoa trunk stream at about 18ka, as recorded in tephra stratigraphy in alluvial fills on abandoned strath terraces. Preliminary map and aerial photography investigation indicates that both knickpoint migration rate and form (diffuse, steep/discrete, waterfall) show considerable variability within the drainage basin. Significant new data on the processes, rates, and styles of knickpoint migration as a function of upstream drainage area (discharge and sediment flux), substrate lithology, and distance of knickpoint migration will be gathered, analyzed, and disseminated. As the Waipaoa is a dramatic example of how dynamic the landscape we live in is, and how susceptible it can be to human disturbance, we are committed to the development of educational modules at all levels, including K-12 instruction.